1. Field of the Invention
The present invention relates to a shield structure for an electronic device for blocking radiation of electromagnetic waves emitted from the electronic device.
2. Related Background Art
An electronic device such as a plasma display panel (hereinafter referred to as a “PDP”) is equipped with a lot of electronic components that generate electromagnetic waves. In recent years, with the digitization and enhanced performance of electronic devices, unnecessary radiation of electromagnetic waves emitted therefrom has increased. As a countermeasure against these unnecessary electromagnetic waves to ensure the electromagnetic compatibility (EMC), a shield structure in which such an electronic device is enclosed with a conductive member has been proposed.
For example, in a plasma display device including a PDP, a film having a conductive layer is disposed at the front side of the PDP and a conductive back cover is disposed at the rear side of the PDP, and the periphery of the film and the periphery of the back cover are connected electrically to each other with a frame-shaped pressing metal member (see, for example, JP 2001-141972 A). More specifically, the pressing metal member has a contact portion having a shape to be fitted along the periphery of the film and a flange portion having a shape to be fitted along the periphery of the back cover. The flange portion of the metal member and the periphery of the back cover are laid one on the other in the thickness direction of the PDP and fastened to each other at plural positions by means of screw members.
In the above-mentioned structure, the flange portion of the metal member and the periphery of the back cover, at positions where they are fastened to each other by the screw members, are brought into surface contact with each other by the fastening force of the screw members. However, gaps are formed between the positions where they are fastened by the screw members, and electromagnetic waves may leak outside the device through the gaps. In order to prevent the leakage, the flatness of both the surfaces of the flange portion of the metal member and the periphery of the back cover that are located at the side to be laid in contact needs be ensured with high accuracy. It is difficult, however, to ensure the flatness of such frame-shaped surfaces with high accuracy in view of both the workability of the surfaces and the difficulty in keeping the surfaces flat. Particularly, in many plasma display devices having large frame-shaped surfaces, it is very difficult to ensure the flatness of such large surfaces with high accuracy.
In view of this, as disclosed in JP 7 (1995)-14697 U, it has been proposed to dispose hemispherical contact portions between the positions where the flange portion of the metal member and the periphery of the back cover are fastened to each other by the screw members on either of them, so that they are reliably connected electrically to each other.
However, if the contact portions as mentioned above are provided on the flange portion of the metal member or the periphery of the back cover, they are in point contact with each other. Thus, the impedance between them cannot be lowered sufficiently.